Mr Rahman, 40, is the joint secretary general of Ms Zia's Bangladesh Nationalist Party and was widely tipped to succeed her before his arrest and imprisonment in March.

Ms Zia and another son, Arafat Rahman, are also in jail pending trial on corruption charges.

About 150 high-profile suspects - including politicians, civil servants and businesspeople - have been arrested on corruption charges since the military-backed caretaker government came to power in January.

They include Ms Zia's great rival, Sheikh Hasina, who is also a former prime minister and leader of the Awami League.

The interim government has vowed to rid the country of corruption before it allows elections to be held in 2008.

For many years Bangladesh was ranked as the world's most corrupt country by the Berlin-based anti-corruption watchdog, Transparency International, but it is now no longer bottom of the list.